1. Field of the Invention
The present invention relates to an optical communication system, and in particular, to a supervisory and control signal transmitting system for use in a system employing optically amplifying repeaters for amplifying attenuated light and transmitting data over a long distance between a transmitting station and a receiving station.
2. Description of the Related Art
Conventionally, when a long-distance optical communication is made through a conventional cable or a submarine cable, an optical signal is converted into an electrical signal by a repeater so as to amplify the electrical signal. Thereafter, the amplified electrical signal is restored to an optical signal. In recent years, optical communication systems according to non-regenerating relay method using optical amplifiers have been developed. The applications of the optical communication systems to submarine optical communication systems have been studied. In the submarine optical communication systems, supervisory and control information of optical amplifiers should be transmitted between an end station and an optical repeater.
In the optical communication systems, supervisory signals for supervising the operational status of repeaters and so forth (signals which represent operational status of units) and signals for controlling status (such as switching an operating unit to a backup unit upon occurrence of a defect, setting a loop-back path, and so forth) should be transmissible through a line which is in operation. According to one of the methods which have been studied, a sub-signal such as a supervisory and control signal is amplitude-modulated to a main signal by several percent so as to transmit supervisory and control information between an end station and a repeater.
In conventional optical communication systems, optical signals are transmitted between two ground stations through an optical cable. Between these stations, repeaters are disposed at predetermined intervals. The repeaters amplify attenuated optical signals, receive control signals (commands) from the ground stations, control the status of units, and transmit supervisory signals (messages) which represent the status of the units to the ground stations according to the control signals. Such supervisory and control signals are burst signals which are generated when required, not always.
FIG. 1A to 1D are schematic diagrams for explaining supervisory and control signals.
FIG. 1A shows an original supervisory and control data signal. A sine wave with a carrier frequency of f1 (see FIG. 1B) is amplitude-modulated according to the value "0" or "1" of each bit constructing the data signal. Thus, a burst shaped sub-signal is generated. When the value of bit is "0", the amplitude of the burst signal is "0" (off). When the value of bit is "1", the amplitude of the burst signal becomes constant (on). The sub-signal containing the supervisory and control signal is superimposed on the main signal (data signal transmitted between ground stations) and transmitted by a transmitting unit or a repeater. FIG. 1D shows a waveform where a conventional main signal is superimposed on a sub-signal.
As shown by the left part of FIG. 1D, the main signal is a broad band signal with very high frequencies (for example, up to 20 GHz) and a fixed amplitude (modulated by transmission data). The broad band main signal is superimposed (amplitude-modulated) with a sub-signal (supervisory and control signal) which contains a carrier with a very low frequency of f1 (for example, of the order of several MHz). Thus, a waveform as shown by the right part of FIG. 1D is generated. In this case, the modulation is performed so that the variation of the amplitude of the main signal by the sub-signal is within several percent of the amplitude of the main signal.
When automatic gain control (AGC) is performed for the supervisory and control signal which is a burst shaped on/off signal so as to stabilize the modulation factor of the main signal, the modulation may not work correctly. In addition, when the AGC raises the gain because of insufficient modulation factor in the signal off state, an oscillation may take place in the signal on state. Thus, the modulation factor is not stable.
A method for solving the problem involved in the transmission of burst shaped signal by an end station and a non-regenerating repeater has been proposed by the applicant of the present invention (as Japanese Patent Application Laid-Open Nos. HEI 3-285378 and HEI 4-8023). In this method, a main signal is amplitude-modulated to sequential waves (referred to as a modulation factor stabilizing signal) with a frequency other than the frequency band of the sub-signal. Thus, the modulation factor of the main signal is stabilized. By controlling the driving amplitude of the other signal with the modulation factor and predetermined drive amplitude, the modulation factor of the output light is stabilized.
When a supervisory and control signal and a modulation factor stabilizing signal are superimposed on a main signal by the above-described method, as shown in FIG. 1D, the optical power of the main signal is lower than that where the modulation is not performed. Thus, the reception sensitivity reduces. In addition, when a signal is relayed by a number of stages of non-regenerating repeaters, the frequency of successive waves should be unique for each repeater. This is because if all the repeaters use the same frequency, they cannot stabilize the modulation factor. Thus, where the lower parts of successive waves which stabilize the modulation factor overlap due to cumulation by each repeater, the optical level of the main signal is considerably reduced, thereby significantly degrading the reception sensitivity.
FIG. 2 1-5 are schematic diagrams for explaining a decrease of the level of the main signal in the proposed method. In the multiple relayed transmission system as shown in FIG. 2 1, a plurality of non-regenerating optical repeaters B, C, . . . , etc. are connected to a transmitter A through a transmission line L. In this transmission system, it is assumed that a signal which is output from the transmitter A is a main signal on which a lower frequency wave (sub-signal) is not superimposed as shown in FIG. 2 2. FIG. 2 3 shows an optical output where successive waves with a frequency of f2 (low frequency) are superimposed (amplitude-modulated) on the main signal in the first non-regenerating repeater B so as to stabilize the modulation factor.
This output is supplied to the second stage non-regenerating optical repeater C. Successive waves with a frequency of f4 (low frequency) shown in FIG. 2 4 which differs from the frequency f2 of the waves for stabilizing the modulation factor of the first repeater B are superimposed on the signal shown in FIG. 2 3 by the second stage non-regenerating optical repeater C Thus, the non-regenerating optical repeater C generates an optical output as shown in FIG. 2 5. In the waveform of the signal shown in FIG. 2 5, at a portion where lower parts of two low frequency waves f2 and f4 are overlapped, the optical level of the main signal is considerably reduced, thereby significantly degrading the reception sensitivity. This problem will further take place whenever a different low frequency wave is superimposed by the third or later stage non-regenerating repeater in the transmission system shown in FIG. 2 1.
As described above, when successive waves with a frequency which differs from the frequency of a supervisory and control signal are superimposed by a transmitter and a plurality of repeaters so as to stabilize the modulation factor, the minimum amplitude becomes (1-n.times..GAMMA.) times that of the main signal on which waves are not superimposed, where .GAMMA. is the modulation factor of successive waves of each repeater; and n is the number of successive waves. For example, when the modulation factor of successive waves of each repeater is 5%, if successive waves are superimposed by 20 repeaters, the modulation factor may become at worst 100%. Thus, multiple-repeater transmission will become difficult.
In the above description, the related art of the transmission method of a supervisory and control signal for use in a non-regenerating repeater system using only optical amplifiers was discussed. In addition, in a long distance communication system, an optically amplifying repeater system which has both optically amplifying repeaters and reproducing repeaters is known. In this system, after an optical signal is relayed by several stages of optically amplifying repeaters, a reproducing repeater which has so-called 3 R reproducing function converts the optical signal into an electrical signal and then amplifies the signal.
In the system which has both the optically amplifying repeaters and the reproducing repeaters, the line switching, investigation of degrade of transmission quality, maintenance work, and so forth will be quickly performed, when a later stage of an end station can detect a defect which takes place in the optically amplifying repeaters.
In a synchronous digital hierarchy (SDH) interface which is used in optical fiber communication networks such as broad band ISDN, an overhead which is transmitted along with a main signal can accord with enhanced maintenance and operation functions (CCITT recommendations G. 707 to G. 709). In the SDH interface, one frame is constructed at intervals of 8 kHz (125 /.mu./second).
FIG. 3 shows an example of the construction of an STM-1 frame which is the lowest format of SDH interface. In the figure, reference numerals 1 and 3 are section overheads SOH. Reference numeral 2 is an AU pointer. Reference numeral 4 is a payload in which data is stored.
The first to third lines of the section overhead (SOH 1) shown in FIG. 3 are used for communications between repeaters and between a repeater and an end station. The fifth to ninth lines of the section overhead (SOH 3) are used for a communication between end stations.
The section overhead SOH used for the communications between repeaters and between a repeater and an end station has various areas used for synchronization, error monitor, order wire (on which sound information is placed), defect evaluation (F1 byte), and so forth. The conventional reproducing repeaters have functions for reading and writing data from and to these areas and for generating a signal with the same frame construction as a main signal if the main signal is lost due to a defect. On the other hand, since the optically amplifying repeaters amplify and relay signals without conversions between optical signals and electrical signals, they cannot perform the overhead process as opposed to the reproducing repeaters.
The defect informing methods of the conventional optically amplifying repeaters can be categorized as 1 a method where each repeater sends the current operational status to an earlier stage end station or a later stage end station through a loop-back according to a command received from an end station (for detail of this method, refer to document 1), 2 a method where a response signal is superimposed on a main signal by finely modulating an exciting LD (for detail of this method, refer to document 2), and 3 a method where a defect is detected by using an exciting light (for detail of this method, refer to document 3).
However, from the view point of reliability of the optically amplifying repeaters which are required to output high power output, it is likely that the reliability of the exciting LD is lower than that of other constitutional parts. Thus, in the conventional method, if the exciting LD becomes defective, the repeater cannot send the related message to later stages of units.
There is also another method where a dedicated light source for a supervisory signal is used (for detail of this method, refer to document 4), However, in the optically amplifying repeater system which has both optically amplifying repeaters and reproducing repeaters, such a problem has not been solved.
The document 1 is "The OS-280M repeater supervisory system", by H. WAKABAYASHI, at. al. The document 2 is "Supervisory and Control Method in Optically Amplifying Relay System (Translated Title)", 1992 Spring Convention, B-944, The Institute of Electronics, Information, and Communication Engineers, Japan. The document 3 is "Supervisory Signal Transmission for Optical Amplifier System", by S. MATSUOKA et. al., 1990 GLOBECOM '90, 903.2. The document 4 is "Study of Supervisory Signal Transmission System for Linear Repeater (Translated Title)", by YAMABAYASHI et. al., B-943, 1992 Spring Convention, The Institute of Electronics, Information, and Communication Engineers, Japan.
Thus, in conventional optically amplifying repeater systems which have both optically amplifying repeaters and reproducing repeaters, since the defect informing system uses a defect informing portion whose reliability is low, a supervisory signal which represents a defect of an optically amplifying repeater is not securely sent to a related end station.